Anti-smudge coatings are of interest for touchscreens to provide a better consumer experience by increasing perception of cleanliness. Touchscreens have become pervasive in society, including phone screens, computer screens, vehicle controls, appliances, and so on. Oil smudges can be problematic for many types of surfaces (e.g., refrigerators), not only transparent surfaces.
Anti-fouling coatings designed to keep surfaces clear of water, oil, mud, and so on are generally desirable to maintain both performance and aesthetics. Many coatings in the anti-fouling category operate on a principle of wetting and dewetting. Such coatings are made of a single component to create water or oil with a high contact angle, causing the oil to bead up and roll off of a surface in order to clear it of unwanted debris. Such coatings perform poorly against oil and grease deposited in a thin layer, typically by means of mechanical rubbing of an oil-containing substrate onto the coating (versus an oil spill for example). After simple rubbing, there is not enough oily material to effectively bead up and roll off the surface.
There are a considerable number of examples of anti-smudge coatings in existence today. Many of these technologies are based on a fluorinated surface that acts to create an oleophobic coating based on a pure dewetting mechanism. Challenges in this area include the need to reduce inclusions to typically less than 50 nanometers in order to minimize light scattering and maintain good visual transparency. Limited commercial success of a genuine oil-absorbing anti-smudge technology is complicated by the need for durability, scalability, and permanence of the anti-smudge properties throughout the lifetime of the coating. The combination of these properties is lacking in current off-the-shelf technology reported in the academic literature.
Current anti-smudge coatings focus on incorporating oil-repelling species, sometimes coupled with deliberate surface roughness.
In Rabnawaz et al., “Graft-Copolymer-Based Approach to Clear, Durable, and Anti-Smudge Polyurethane Coatings”, Angew. Chem. 2015, 127, 6616-6620, small discrete fluoro domains in a sea of highly crosslinked polyurethane are employed in order to produce a transparent anti-smudge coating. The approach employed is to functionalize a polyurethane polyol precursor with monofunctional PFPE (Krytox™ oils). When blended with unfunctionalized polyol and crosslinker, small domains of fluoro-containing elements are created.
In Rabnawaz et al., “Fluorine-Free Anti-Smudge Polyurethane Coatings”, Angew. Chem. 2015, 127, 12913-12918, transparent anti-smudge polyurethane coatings are created without the use of fluoro components. To achieve this, polydimethylsiloxane (PDMS) was used as the hydrophobic domain and was grafted onto the polymer backbone to eliminate macrophase separation which would cause haze. The approach employed is to functionalize a polyurethane polyol precursor with monofunctional PDMS to avoid expensive fluoro-containing reagents. When blended with unfunctionalized polyol and crosslinker, small domains of PDMS elements are created.
Rabnawaz et al. describe structured polymer coatings containing oleophobic regions through the use of fluorinated domains. Their work provided transparent coatings which successfully passed anti-smudge benchmark tests such as contact angle measurements with oil and water. These coatings solely rely on the oil repellency of fluorinated or polydimethylsiloxane (PDMS) species and find success at the macroscale. These coatings perform based on a dewetting mechanism, which will not be able to remove small amounts of remaining oil, as discussed above. Furthermore, Rabnawaz et al. use a graft copolymer approach to achieve microphase-separated structured domains which is not easily scalable to commercially relevant quantities.
In Hikita et al., “Super-Liquid-Repellent Surfaces Prepared by Colloidal Silica Nanoparticles Covered with Fluoroalkyl Groups”, Langmuir 2005, 21, 7299, liquid repellent coatings are made through optimizing surface roughness and chemistry. The coatings were prepared via sol-gel polycondensation using colloidal silica and fluorosilanes added to increase oil repellency. The approach demonstrates the effect of surface roughness on wetting properties, showing increased water and oil contact angles at an optimum surface roughness. The manuscript claims scalable, durable, transparent films—however, the coating is spin-coated onto the substrate resulting in a very thin (˜500 nm) film. This application technique would be difficult to scale commercially to a variety of substrates. Additionally, this technology relies solely on the structure of the surface for optimal performance. Wear of the coating over time would negate the positive surface roughness effects.
In Tuteja et al., “Designing Superoleophobic Surfaces”, Science 2007, 318, 1618, superoleophobic surfaces are created through optimizing surface structure and chemistry. A combination of deposition and electrospinning created surfaces with advancing and receding contact angles>130° for octane. The approach demonstrates the effect of surface chemistry and structure on dewetting properties, specifically focused on superoleophobicity. Tuteja et al. show the Cassie and Wenzel hypotheses in action, successfully creating oleophobic silicon surfaces. The surface techniques would be difficult to scale commercially to a variety of substrates and this technology relies solely on the structure of the surface for optimal performance. Wear of the coating over time would negate the positive surface roughness effects.
Surface and/or coating roughness is another variable exploited to increase oleophobicity. Literature, as described by the work of Tuteja et al. and Hikita et al. above, shows that rough surfaces commonly shed liquid contaminants (oil) better than smooth surfaces. A pitfall of this strategy is that the improved hydrophobicity and oleophobicity is strictly dependent on the surface structure and often requires deposition techniques not easily scalable to large quantities or a variety of substrates. As the coating is used and wears or erodes over time, the effect is diminished.
Oleophilic wipes are on the market, designed to remove smudges from various surfaces such as touchscreens. However, these wipes are not a passive solution that is built into the coating itself.
What is needed is an oil-smudge-resistant material and coating that is durable, scalable, and permanent. The coatings need to remain effective over the lifetime of the coating. Optically transparent, oil-smudge-resistant coatings are important for a number of applications.